Conventional holographic memory systems normally employ a page-by-page storage approach. An input device such as SLM (spatial light modulator) presents recording data in the form of a two dimensional array (referred to as a page), while a detector array such as CCD camera is used to retrieve the recorded data page upon readout. Other architectures have also been proposed wherein a bit-by-bit recording is employed in lieu of the page-by-page approach. All of these systems, however, suffer from a common drawback that they require the recording of huge quantities of separate holograms in order to fill the memory to its full capacity. A typical page-oriented system using a megabit-sized array would require the recording of hundreds of thousands of hologram pages to reach the capacity of 100 GB or more. Even with the hologram exposure times of millisecond-order, the total recording time required for filling a 100 GB-order memory may easily amount to at least several tens of minutes, if not hours. Thus, another conventional holographic ROM system has been developed, where the time required to produce a 100 GB-order capacity disc may be reduced to under a minute, and potentially to the order of seconds.
FIG. 1 shows a conventional holographic ROM system including a light source 10; a shutter 12; mirrors 14, 28, 34, 40; HWPs (half wave plates) 16, 24, 36; spatial filters 18, 30, 42; lenses 20, 44; a PBS (polarization beam splitter) 22; polarizers 26, 38; a conical mirror 32; a data mask 46; and a holographic medium 48.
The light source 10 emits a laser beam with a constant wavelength, e.g., a wavelength of 532 nm. The laser beam, which is of only one type of linear polarization, e.g., P-polarization or S-polarization, is provided to the mirror 14 via the shutter 12 which is opened to transmit the laser beam therethrough when recording data on the holographic medium 48. The mirror 14 reflects the laser beam to the HWP 16. The HWP 16 rotates the polarization of the laser beam by θ degree (preferably 45°). And then, the polarization-rotated laser beam is fed to the spatial filter 18 for removing noises included in the polarization-rotated laser beam. And then, the polarization-rotated laser beam is provided to the lens 20 for expanding the beam size of the laser beam up to a predetermined size. Thereafter, the expanded laser beam is provided to the PBS 22.
The PBS 22, which is manufactured by repeatedly depositing at least two kinds of materials each having a different refractive index, serves to transmit, e.g., a horizontally polarized laser beam, i.e., S-polarized beam, along a S1 path and reflect, e.g., a vertically polarized laser beam, i.e., P-polarized beam, along a S2 path. Thus the PBS 22 divides the expanded laser beam into a transmitted laser beam (hereinafter called a reference beam) and a reflected laser beam (hereinafter called a signal beam) having different polarizations, respectively.
The signal beam, e.g., of a S-polarization, is reflected by the mirror 34. And then the reflected signal beam is provided to the mirror 40 via the HWP 36 and the polarizer 38 sequentially. Since the HWP 36 can rotate the polarization of the signal beam by θ′ degree and the polarizer 38 serves to pass only a P-polarized signal beam, the HWP 36 and the polarizer 38 can regulate the amount of the P-polarized signal beam arriving at the mirror 40 by changing θ′. And then the P-polarized signal beam is reflected by the mirror 40 toward the spatial filter 42 for removing imperfectly polarized components of the signal beam and allows only the purely P-polarized component thereof to be transmitted therethrough. And then the signal beam with perfect or purified polarization is provided to the lens 44 for expanding the beam size of the signal beam up to a preset size. Thereafter, the expanded signal beam is projected onto the holographic medium 48 via the data mask 46. The data mask 46, presenting data patterns for recording, functions as an input device, e.g., a spatial light modulator (SLM).
Meanwhile, the reference beam is fed to the mirror 28 via the HWP 24 and the polarizer 26 sequentially. Since the HWP 24 can rotate the polarization of the reference beam by θ″ degree and the polarizer 26 serves to pass only a P-polarized reference beam, the HWP 24 and the polarizer 26 can regulate the amount of the P-polarized reference beam arriving at the mirror 28 by changing θ″. Therefore, the polarization of the reference beam becomes identical to that of the signal beam. And then the mirror 28 reflects the P-polarized reference beam toward the spatial filter 30 which removes imperfectly polarized components of the reference beam and allows only the purely P-polarized component thereof to be transmitted therethrough. And then the reference beam with perfect or purified polarization is projected onto the conical mirror 32 (the conical mirror 32 being of a circular cone having a circular base with a preset base angle between the circular base and the cone), which is fixed by a holder (not shown). The conical mirror 32 reflects the reference beam toward the holographic medium 48. The incident angle of the reflected reference beam on the holographic medium 48 is determined by the base angle of the conical mirror 32.
FIG. 2 shows a conventional method for recording data in the holographic ROM, in detail. As shown in FIG. 2, the holographic medium 48 is a disk-shaped material for recording the data patterns. The data mask 46, also having a disk shape with a similar size to that of the holographic medium 48, provides the data patterns to be stored in the holographic medium 48. By illuminating the data mask 46 with a normally incident plane wave, i.e., the signal beam, and by using the reference beam incident from the opposite side to record holograms in the reflection geometry shown in FIG. 2, the diffracted pattern is recorded on the holographic medium 48. A conical beam shape is chosen to approximate the plane wave reference beam with a constant radial angle at all positions on the disc, such that the hologram can be read locally by a fixed-angle narrow plane wave while the disc is rotating during playback. Furthermore, an angular multiplexing can be realized by using the conical mirror 32 with a different base angle.
However, in case a center 46a of the data mask 46 and a center 48a of the holographic medium 48 are misaligned, i.e., the data mask 46 and the holographic medium 48 are not coaxially aligned, as shown in FIG. 3, the data patterns are recorded along tracks 48c which are also misaligned with the holographic medium 48. Then, since the recorded tracks 48c are not matched with tracks 48b during playback, it becomes difficult to reproduce the data patterns during playback. Therefore, the alignment of the data mask 46 and the holographic medium 48 should be executed before the data patterns are recorded.